In the semiconductor industry, metal layers may be deposited on semiconductor wafers by electroplating processes. The layers are formed of such metals as gold, copper, tin and tin-lead alloys, and they typically range in thickness from 0.5 to 50 microns. The general nature of the process is well-known. The wafer is immersed in an electrolytic bath containing metal ions and is biased as the cathode in an electric circuit. With the solution biased positively, the metal ions become current carriers which flow towards and are deposited on the surface of the wafer.
There are several criteria that need to be satisfied in such a system. First, the thickness of the layer must be as uniform as possible. Second, the layer is often deposited on a surface which has narrow trenches and other circuitry features that must be completely filled, without any voids. Third, for economic reasons the layer must be formed as rapidly as possible.
Assuming that the metal is to be deposited on a nonconductive material such as silicon, a metal "seed" layer, typically 0.02 to 0.2 microns thick, must initially be deposited, for example by physical or chemical vapor deposition, before the electroplating process can begin. The electrical contacts to the wafer are normally made at its edge. Therefore, since the seed layer is very thin, there is a significant resistive drop between the points of contact on the edge of the wafer and the center of the wafer. This is sometimes referred to as the "terminal effect". Assuming that the system is operating in a regime where the plating rate is determined by the magnitude of the current, the plating rate is greater at the edge of the wafer than at the center of the wafer. As a result, the plated layer has a concave, dish-shaped profile. Once the seed layer has been built up by the plated layer, the terminal effect diminishes and the plated layer is deposited at a more uniform rate, although the top surface of the plated layer retains its dish-shaped profile.
One factor which influences the plating rate and thickness profile is the rate at which the metal ions move near the surface of the wafer, often referred to as the "mass transfer rate". When the mass transfer rate is high and the current level is low, all areas of the surface of the wafer are supplied with an ample quantity of ions, and the mass transfer rate has no effect on the thickness profile of the layer. Conversely, when the mass transfer rate is low and the current is high, the mass transfer of the metal ions to the wafer surface becomes the critical factor in determining the rate at which the metal is deposited. The process is then called "mass transfer limited". In this situation, variations in the rate of mass transfer from one point to another on the wafer surface will produce corresponding variations in the plating rate. For example, if the rate of mass transfer at the center of the wafer is high compared to that near the edge of the wafer, the deposited layer can be expected to have a greater thickness at the center of the wafer than near its edge.
The ability of the plated layer to fill features in the underlying surface generally depends on the size of the plating current. In most cases, there is an optimum current for filling features of a given size and aspect ratio with a given metal. For example, if filling is ideal at a current density of 15 mA/cm.sup.2, the initial plating should proceed at that current density.
The terminal effect can be overcome by the use of insulating shields which shift the current away from the portions of the wafer nearest to the electrical contacts. Such shields are described, for example, in U.S. Pat. No. 3,862,891 to Smith and U.S. Pat. No. 4,879,007 to Wong.
The problem with using shields is that they remain in place even after the thickness of the metal layer has increased to the point where the terminal effect is no longer present.
Accordingly, there is a clear need for a technique which overcomes the terminal effect and has good feature filling qualities yet allows the metal layer to be plated at a rapid rate.